1. Field of the Invention
This invention relates to radiation scanners and, more particularly, to a scanner employing an array of infrared detectors in combination with signal compression and averaging circuitry for improved uniformity in reproduction of scenes having substantial uniformity of temperature in each of a plurality of bar-shaped regions parallel to a line of scan.
2. Description of Related Art
One well-known form of infrared imaging system employs an array of infrared detectors and a lens for directing rays of radiation from a portion of a scene being viewed upon the array of detectors. Typically, the detectors are arranged along a line and are scanned in a direction perpendicular to the line, the line of detectors providing the width of a swath being swept during a single line of scan.
In one application of considerable interest, the scene being imaged is viewed on a display wherein the scan lines are parallel to a horizontal axis of the display while the line of detectors is parallel to a vertical axis of the display. Typical subject matter of the foregoing scene would include both sky and land. As viewed by infrared detectors, the sky presents subject matter which is of a lower temperature than the subject matter presented by land. Also included within the typical scene would be objects of higher temperature, such as aircraft, within the sky, and cooler subject matter such as a lake or river on the land. Buildings, roadways, trees, and other foliage may also be present as further subject matter within the scene.
The foregoing portions of the subject matter are characterized by infrared radiation at differing temperatures characteristic of the subject matter in response to illumination by the sun or in response to some other source of heat. The detectors produce electric signals of varying amplitudes dependent upon the temperature of the subject matter of the portion of the scene being viewed by respective ones of the detectors. Thereby, a display responsive to the magnitudes of the detector signals can present the forms of the various objects in the scene being viewed.
In viewing the foregoing scene, the scanner would be oriented relative to the scene such that the line of scan would be parallel to the horizon. As a result, individual swaths of the scene swept by corresponding ones of the detectors would be predominantly cool or predominantly warm depending on whether a detector was scanning a portion of the sky or a portion of the land. A path scan which traversed both a building and foliage, or a building and the lake, would view subject matter which, on the average, has a temperature between that of the sky and that of the land.
A problem arises in the operation of such a scanner due to a variation among the detectors in their responsivity to incident infrared radiation. Thus, two detectors receiving the same radiation may produce output signals of differing amplitudes. This further compounds the foregoing problem of the dynamic range in that portions of the image scanned by certain ones of the detectors may be overly intense in their presentation on the display. This intensifies the problem of streaking.
One solution to the problem of differing detector responsivity has been the use of individual alignment circuits incorporating manually adjustable potentiometers for each of the detectors. The solution is disadvantageous because of a requirement for excessive operator time in the manual adjustment of the potentiometers in a scanner of many, possibly in excess of one hundred, detectors in the detector array.
In the construction of the typical scanner, all of the detectors are placed within a single chamber which is maintained by cooling equipment at a low temperature. When replacement of a number of the detectors is required, the entire array is replaced in which case all of the potentiometers must be reset manually for alignment of the scanner.
A further solution to the dynamic range problem has been the scaling of all signals presented to the display as by use of automatic gain control employing the injection of a pilot signal into the field of view of each of the detectors. Such operation involves unwanted complexity such as additional optics for injection of an infrared pilot signal.